The invention is especially concerned with measuring the speed of an object as it moves across a line-of-sight when the sight itself is moving, e.g. panning. Such a situation arises with targeting apparatus which comprises a rangefinder (laser, radar etc.) and a ballistic microcomputer which, in use, processes data such as the type of ammunition, distance to target and wind speed, to compute the superelevation and lead angle required to place the round on target. It is desirable for the crossing speed of the target to be taken into account to allow for the fact that the target changes position after the round is fired. The weapon can then be aimed to lead the target position by a compensating amount.
Various approaches might be used to determine crossing speed. For example, the gunner could hold the sight still and time target motion relative to a scale in the sight. This lengthens the engagement time and imposes a steadiness requirement which may be hard to meet with a hand-held weapon.
U.S. Pat. Nos. 4,253,249 and 4,695,256 disclose weapons training simulators in which movement of the target across the line-of-sight is computed for the purpose of assessing the accuracy with which the gunner aimed the weapon. In the system disclosed in U.S. Pat. No. 4,253,249, crossing speed is obtained by measuring the movement of the gun turret as the gunner tracks the target. This technique would not be entirely satisfactory "in the field", since it would require lengthy tracking, gunner skill, a ground reference and an angle transducer. In the system disclosed in U.S. Pat. No. 4,695,256, crossing speed is obtained by detecting laser "echoes" returned from the target. This would not be entirely satisfactory "in the field" because it would require lengthy engagement time, gunner skill, and steadiness.
U.S. Pat. No. 4,404,890 issued September, 1983 (McAlpine) discloses a fire control system in which a computer calculates the required gun lead angle. Two images are superimposed within an optical sight. One is of the field of view and the other is of a pattern which is movable relative to the display by adjustment devices controlled by the operator. A disadvantage of this system is the need for manual adjustment by the operator.